1. Field of the Invention
The invention is directed to a heat-sensitive recording material comprising a substrate having at least one heat-sensitive recording layer on the front side thereof, this heat-sensitive recording layer in turn having at least one dye precursor and at least one color acceptor which react with one another under the action of heat to form color. According to the invention, the heat-sensitive recording layer is covered by a protective layer. Further, the heat-sensitive recording material has a barrier coating which is capable of protecting the heat-sensitive recording layer against penetration into the substrate side of the heat-sensitive recording layer by chemicals which can cause uncontrolled desensitizing reactions and color reactions. These chemicals which can cause uncontrolled desensitizing reactions and color reactions in the heat-sensitive recording layer are possibly constituents of tinctures according to the invention which are introduced into the substrate and/or applied to at least one side of the substrate as an authenticating security feature.
2. Description of the Prior Art
A heat-sensitive recording material having a barrier coating within the material is known in principle, for example, from EP 0 938 419 B1. In this case, a coating applied to the back of the substrate of the known recording material has a mixture containing:                starch,        an acrylate copolymer which does not include any styrene components or vinyl acetate components and which has a film-forming temperature of less than 5° C., preferably less than 2° C.,        and an alkaline catalyst, e.g., calcium carbonate.The back-coating of the known heat-sensitive recording material is intended to ensure a good barrier effect against substances used in offset printing and flexographic printing, particularly organic solvents, and against plasticizers, oils and fats.        
A heat-sensitive recording material of comparable construction is also known from EP 2 033 802 A1, according to which the recording material has a substrate to which is applied on the front side thereof a heat-sensitive recording layer covered by a protective layer and which has on the back side a backcoat which, in addition to pigments and binders, includes a polyurethane-based component acting as cross-linking agent.
Both of these heat-sensitive recording materials have proven to be of commercial merit and offer excellent solutions to the problem of a backcoat protecting against plasticizers, oils and fats. However, neither publication suggests a way to protect the heat-sensitive recording layer against chemicals which can cause either an unwanted color-forming reaction in the recording layer or fading or extinguishing of a printed image which is to be formed, or which is formed, in the recording layer, these chemicals—also in large amounts—being contained in the substrate and/or applied to at least one side of the substrate. Isolation of this problem was achieved after lengthy analysis of numerous defective products which made it necessary when formulating the problem to also put to the test the common basic processes for producing heat-sensitive recording materials.
In formulating a problem of this kind, the tightness of layers against organic solvents in particular plays a prominent role. Beyond this, the inventors realized after numerous preliminary tests that many of the known or obvious formulas for making a protective coating very often led to processing difficulties in handling during production due to rheological peculiarities of the coating compositions which are generally self-crosslinking. These processing difficulties lead particularly to coating defects because of excessively highly viscous coating composition constituents and to a gradual clogging in the pipelines to the coating units.
A composition of a heat-sensitive recording material approaching the aimed for solution to the problem described in the following is shown particularly in U.S. Pat. No. 5,804,529 A which provides for embedding a heat-sensitive recording layer between two layers based on polyvinyl alcohol or on ethylene vinyl alcohol copolymer (EVOH). Although the goal of this known suggestion is to provide a heat-sensitive recording material having excellent light fastness, while other effects of this known suggestion are not mentioned in the document, this suggestion, as the prior art coming closest, is particularly significant.
To the extent that they are technically comparable, the disclosures of U.S. Pat. No. 6,150,067 A should also be mentioned, whereas the disclosures of U.S. Pat. No. 5,641,724 A—a reversible heat-sensitive recording compound—only disclose remote prior art.
The documents evaluated thus far say nothing about security features possibly being provided in the heat-sensitive recording materials and consequently also nothing about problems occurring in this respect or how these problems may be solved.
In this regard, security features by which the genuineness of a recording material can be proved are made possible as proof of authenticity for documents in the form of passive inspection, for example, through watermarks. By watermark is meant, generally, a mark in the paper which is generated through different paper thicknesses. A distinction is made between true watermarks which are produced by displacement (so-called light watermarks) or by concentration (so-called shaded watermark) of the fiber pulp using, for example, a dandy roll in the wire section of a paper machine, impressed watermarks, also called Molette watermarks, which are produced by impressing the paper while still wet in the press section of a paper machine, and, finally, imitation watermarks which are made either by imprinting the finished paper outside the paper machine with a colorless varnish or by stamping the finished paper outside the paper machine.
Heat-sensitive recording materials with authenticating security features in the form of watermarks are generally known. For example, EP 0 844 097 A1 discloses, as first security feature for a heat-sensitive recording material in the form of an imitation watermark, a latent image which is imprinted on the back side of the recording material and is produced by a security ink containing a fluorescent reagent. The security ink contains a water-repelling agent to form a second security feature in the form of a waterproof image on the back side of the heat-sensitive recording material. The security ink containing the fluorescent reagent, which is used as pigment or dye, and the water-repelling agent is contained or dispersed in an aqueous carrier which, in addition to these components, can also contain a binder.
A disadvantage to this suggestion consists in that counterfeiting or copying can be accomplished fairly easily owing to what must really be characterized as a simply structured security ink. The reason for this simple structuring is that security inks having a complex structure are mostly poorly compatible with the chemical processes taking place during the formation of character images in the heat-sensitive recording layer; in the worst case, security inks of complex structure impede or even prevent these text-forming processes.